Drying coated substrates, such as webs, requires heating the coating and then removing the evaporated liquid. Convection, conduction, and radiation are used to heat coated webs. Applied convection or forced gas flow is used to remove the evaporated liquid. Applied convection is defined as convection produced by the input of power and caused intentionally. It excludes convection caused merely by web movement, natural convection, and similar, unintentional forces. In some instances where the vapors are non-toxic, such as water evaporation, the vapor is removed by flashing off into the ambient atmosphere.
In conventional drying technology, large volumes of gas, inert or not, are required to remove evaporated liquid from the gas/liquid interface. These dryers require large gaps between the coated web being dried and the top of the drying enclosure to accommodate the large gas flows. Drying is governed at the gas/liquid interface by diffusion, boundary layer air from the moving web, vapor concentrations, and liquid to vapor change-of-state convection, among other factors. These phenomena occur immediately above the coated web, typically within 15 cm of the surface. Because conventional dryers have a large gap above the coated web, and they can only control the average velocity and temperature of the bulk gas stream, they have limited ability to control these phenomena near the gas/liquid interface.
The vapor concentrations in these bulk gas streams is kept low, typically 1-2%, to remain below the flammable limits for the vapor/gas mixture. These large gas flows are intended to remove the evaporated liquid from the process. The expense to enclose, heat, pressurize, and control these gas flows is a major part of the dryer cost. It would be advantageous to eliminate the need for these large gas flows.
These gas streams can be directed to condensation systems to separate the vapors before exhausting, using large heat exchangers or chilled rolls with wiping blades. These condensation systems are located relatively far from the coated web in the bulk gas flow stream. Due to the low vapor concentration in this gas stream, these systems are large, expensive, and must operate at low temperatures.
It would be advantageous to locate the condensation systems close to the coated substrate where the vapor concentrations are high. However, conventional heat exchangers would drain the condensed liquid by gravity back onto the web surface and affect product quality unless they were tilted or had a collection pan. If they had a collection pan they would be isolated from the high concentration web surface. If they were tilted dripping would probably still be a problem. Also, conventional heat exchangers are not planar to follow the web path and control the drying conditions.
U.S. Pat. No. 4,365,423 describes a drying system which uses a foraminous surface above the web being dried to shield the coating from turbulence produced by the large gas flows to prevent mottle. However, this system does not eliminate applied convection, requires using secondary, low efficiency solvent recovery, and has reduced drying rates. Also, because of the reduced drying rates, this patent teaches using this shield for only 5-25% of the dryer length.
German Offenlegungeschrift No. 4009797 describes a solvent recovery system located within a drying enclosure to remove evaporated liquid. A chilled roll with a scraping blade is placed above the web surface and removes the vapors in liquid form. No applied convection removes the evaporated liquid. However, the roll is only in the high vapor concentration near the surface for a short section of the dryer length. This does not provide optimal control of the conditions at the gas/liquid interface. In fact as the roll rotates it can create turbulence near the web surface. Also, this system can not adapt its shape to the series of planar surfaces of the coated web as it travels through the dryer. Therefore, the system can not operate with a small, planar gap to control drying conditions and can not achieve optimum condensing efficiency.
There is a need for a system for drying coated substrates which provides improved control of the conditions near the gas/liquid interface, which eliminates the need for applied convection to remove the evaporated liquid, and which improves the efficiency of the condensation vapor recovery systems.